1. Field of the Invention
The invention relates, in a conveyor system, to a device for retarding stacks of sheets, each consisting of at least one sheet, intended, for example, to be fed together into a stream, train or chain of overlapping stacks.
2. Prior Art
The next stack of sheets in a sequence of overlapping stacks fed onto a conveyor is generally passed from above onto the stream of stacks which has already been formed and which is travelling at a continuous conveyor speed, at a feed rate greater than the conveyor speed of the stream of stacks at the transfer point, so that the leading end of this new sheet stack overtakes the trailing end of the stream of stacks and is thus deposited overlapping the stream. Once the overlap dimension desired, determined by the degree of overlap, is achieved, the sheet stack most recently fed must be slowed to the conveyor rate referred to, so that it will run synchronously with the stream of stacks as its last component. A braking element forming a braking gap in conjunction with the belt carrying the stack stream, the gap width of which may be adjusted to the stack stream thickness which will vary as a consequence of the overlap, best provides this retardation. The stack thickness of the stack stream can change, particularly in its longitudinal direction or direction of transport, in steps as a result of the overlap, either repeatedly increasing or decreasing or almost continuously. The braking system supplies the braking force by acting on the stack sheet as it is fed with a frictional or braking pressure so that this stack is retarded to the conveyor speed by means of the sliding friction against the stack stream on the one hand and against the braking element on the other. The braking element referred to is provided to brake the stack sheet fed into it by acting, in particular, on its leading end in the direction of feed or transport. Instead of this, or in addition to this, it is also useful, in particular in the case of large sheet lengths, for the sheet to be retarded at its trailing end, so that the sheet is stretched out in the direction of feed or transport under braking. A suction device is used for this purpose, causing the sheet concerned to adhere to the conveyor belt or the individual sheets of the sheet stack to adhere to one another, thereby decelerating them from the higher feed rate to the transport speed of the conveyor. However, the more sheets laying on top of one another in the sheet stack, or the more impermeable the sheets are to air, as is the case, for instance, with coated papers, the more difficult it will be for the suction air to penetrate the individual sheets.
For these reasons it is necessary, particularly in the case of high feed and transport rates or high machine speeds, to provide an additional or different means of retardation, preferably acting on the leading end of each sheet as it is fed. This retardation is best applied mechanically in the manner described by braking friction in the braking gap, which lies for this purpose at the end of a tapered gap narrowing at an acute angle in the direction of feed or transport. It is extremely difficult to adjust the gap width in relation to the particular stack thickness of the constantly varying stack stream currently passing since, on the one hand although a good retarding action is achieved with inadequate gap width, impressions are frequently left on the very sensitive surface of the sheet stacks, and on the other hand, although no impressions are left if the gap width is too great, one or several sheets of a sheet stack may slip through the braking gap or under the braking element with reduced retardation by comparison with the other sheets because of the inadequate retarding action, thereby causing breakdowns. These difficulties are further compounded by the variable stack thickness and also by the fact that the difference between insufficient and excessive gap width is very small.
Attempts have already been made to circumvent these difficulties by making the gap width slightly smaller than the stack height, and allowing this difference to be accommodated by the elastic resilience of the belt. In this case, however, particularly when coated papers are handled, pressure points in the form of strip markings cannot practicably be avoided, as the gap width of the retarder gap can only be adjusted elastically directly by the counter pressure of the stack stream or variably against a preset load. This load has to be relatively high for the sake of rapid adjustment.